The present application is based on and claims priority from Japanese Patent Application Hei 11-340558 filed Nov. 30, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a liquid-cooled vehicle rotary electric machine driven by an internal combustion engine mounted in a vehicle, such as a passenger car or a truck.
2. Description of the Related Art
Recently, it has been considered to use a vehicle AC generator for engine-idling-stop system or an engine-torque assisting system.
U.S. Pat. No. 4,955,944 proposes a liquid-cooled rotary electric machine that has a liquid-cooled frame and a stator core fixedly fitted to the frame.
JP-A-11-146606 proposes a liquid-cooled vehicle rotary electric machine in which coil ends of a stator winding are covered with insulation resin to be cooled by cooling liquid. It is necessary to cool the stator winding by cooling liquid because the coil ends is subject to a high temperature due to its poor heat conduction.
JP-B2-2927288 proposes a vehicle AC generator in which each of a plurality of U-shaped conductor segments (hereinafter referred to as U-segment) is inserted into a pair of slots and each of the ends of the inserted U-segments is serially connected to another to form a stator winding.
As the engine idling speed has been decreasing, it has been demanded to increase electric power. On the other hand, an engine compartment has been made more compact, and a more compact vehicle AC generator has been required. The above demands have necessarily increased the maximum current or the current density of the stator winding of the generator and temperature thereof. In other words, cooling performance of the stator winding is one of decisive factors of the size of a rotary electric machine.
In a generator-motor that has a motor-function to be used for an engine-idling stop system or an engine-torque assist system, it is necessary to supply much more input current to the stator winding thereof than the output current to generate by the stator winding. As a result, the stator winding of the generator-motor presents much more serious temperature-rise problems than ordinary generators.
In order to improve cooling performance of conventional open-type air-cooled vehicle AC generator, it is possible to adopt the above liquid-cooled frame. It was found that the temperature of the stator of the rotary electric machine having the liquid-cooled frame does not become as low as the ordinary rotary electric machine, although the former has more complicated structure. This is one of major reasons why the rotary electric machine having the liquid-cooled frame has not been put into practical use.
The other liquid-cooled vehicle rotary electric machine having multi-layered coil ends that are cooled by cooling liquid via a resinous insulator has also the following problems. It is very difficult to fill resinous material into gaps of the coil ends that are not evenly distributed. Therefore, the temperature of the conductors inside the coil ends becomes so high that the resinous material cracks due to a difference in thermal expansion between the resinous material and the conductors of the coil ends. As a result, cooling liquid (or water) may get into the inside of the coil ends through the cracks, causing short-circuiting or grounding.
There is the highest heat-transmission resistance between portions of conductor segments inside the slots and the insulators. Because the stator winding of the conventional liquid-cooled rotary electric machine is formed of round-wire coils wound one after another by a winding machine, it is very difficult to increase the conduction space factor of the slot to a certain higher value or to reduce dead spaces from the slot.
When the winding is wound by a winding machine in the manner described above, the coils overlap each other in a plurality of layers and swell in the radial direction. However, the number of conductors of the coil ends has to be limited because the spaces at opposite ends of the stator core are limited for the coil ends. This also obstructs to increase in the space factor.
Because considerably large portions of the slots are occupied by the dead spaces, the temperature of the stator winding becomes much higher than the temperature of the insulators and the stator core although a liquid-cooled frame is provided.
The present invention has been made in view of the above problems. Therefore, a main object of the invention is to provide a compact and powerful liquid-cooled rotary electric machine.
A main feature of the invention is a rotary electric machine that includes a liquid-cooled frame in which liquid flows and a stator core that is fixedly fitted to the inner periphery of the frame. A stator winding is formed of a plurality of serially connected U-segments. The stator winding having the serially connected U-segments provides excellent cooling performance. As a result, a drastically smaller size-to-current ratio can be achieved.
There is a heat transmission channel in the rotary electric machine having a liquid-cooled frame between the heat generating stator winding and the cooling liquid. Heat is transmitted through the channel from the stator winding (including resinous insulation coating), insulators disposed between the stator winding and surfaces of slots, a stator core, a frame and the cooling liquid.
The U-segment has much larger cross-sectional area than any other round wire conductor. The U-segment also has a rectangular cross-section that is fitted to the cross-sectional shape of the slot with very small spaces being interposed.
As the cross-sectional area is, increased, the number of portions of the U-segment disposed in the slots (hereinafter referred to as the in-slot portions) can be reduced. The rectangular cross-section of the U-segment makes each in-slot portion thereof close contact with the inner wall of the slot via an insulator, thereby reducing the heat-transmission resistance.
Because of a large conduction space factor of the slot, the stator winding at the rotary electric machine according to the invention has a large cross-sectional area, which provides a low resistance of the stator winding and low temperature thereof and a high current capacity.
Because the space necessary for the coil-ends of the serially connected U-shaped conductor-segment type stator winding is small, the conduction space factor of the slot is not limited by the coil-end space, and the temperature of the stator winding can be reduced further.
For example, the conduction space factor of the slot of a conventional winding type stator is less than 50%. This is because the cross-sectional area cannot be increased due to difficulty in the winding process.
If the cross-sectional area of the slot of the stator is the same, the heat capacity of the stator winding of the rotary electric machine according to the invention can be made much larger than the conventional rotary electric machine. This can suppress a temperature rise even if the stator winding is supplied with a large amount of current in a short time.
Because the cross-sectional factor of the conductor segment increases, the rigidity thereof increases. The coil ends can be shaped evenly with distances between the inner periphery of the frame and the coil ends become even. Therefore, it is possible to reduce the diameter of the frame, so that the generator can be made compact.
According to another feature of the invention, the rotary electric machine is enclosed liquid-tightly. Therefore, the stator winding becomes resistant to environmental severe conditions.
According to another feature of the invention, each of the in-slot portions is closely fitted to one of the plurality of slot via an insulator. This prevents excessive temperature rise of a limited area of the in-slot portions.
According to another feature of the invention, the number of the slots is larger than the product of the number of the magnetic poles and the number of the phase of the stator.
Because the coil ends do not closely overlap one another in the radial direction, a high conduction space factor can be provided. Because the conductor segments do not overlap one another in the circumferential direction, they can closely contact the inner wall of the slots. Therefore, as the number of the slots increases, the inner surface area of the slots increases without decrease in the space factor or the contact area of the in-slot portions with the slot. Thus, an amount of the heat dissipation can be increased, and the temperature rise of the stator winding can be suppressed more effectively.
According to another feature of the invention, each of the coil ends is separated from each other. Therefore, heat-conductive resinous material can be suitably filled in the gaps between the conductor segments of the coil ends.
Further, the insulator material does not directly contact cooling water. Therefore, there would be no chance of short-circuiting or grounding even if it cracks.
According to a further feature of the invention, the stator winding can be supplied with larger current at a motor mode than current generated at a generator mode, while suppressing the size increase and temperature rise of the stator winding.
A generator-motor for a vehicle operates as a motor to start an engine or assist an engine to accelerate the vehicle. The maximum motor current becomes a number of times as much as the maximum output current when it operates as a generator. Such maximum motor current is supplied in a comparatively a short period of time. Therefore, it is very important to suppress temperature rise of a stator winding while the vehicle generator-motor is starting or assisting the engine, in such a short period of time.
The above-stated serial U-shaped-conductor stator winding has a very high conduction space factor, very low heat resistance of the in-slot portions, and a large mass of the in-slot portions. Therefore, the stator winding can stand very large motor current in a short time.